Abstract

Igneous activity at Independence volcano, a major center in the Eastern Absaroka Belt of Montana, occurred between 92 and 84 m.y. ago. Magma series from high-alumina tholeiitic basaltic andesite through shoshonite to high-K dacite and from high-magnesium andesite to banakite are recognized. Major-and trace-elememt compositions of these rocks are modeled by precipitation of observed phenocryst phases. Crystallizing assemblages are dominated by plagioclase and include two pyroxenes and magnetite. Parental rocks of each suite are LILE-enriched, with strong LREK-enrichment (LaN/YbN = 10–15) but do not have high alkali/alkaline-earth ratios (Rb/Sr = 0.020–0.028). Although superficially similar to those observed in many orogenic volcanics, trace-element patterns of Independence volcanics show certain distinctive characteristics, particularly in extreme enrichments in alkaline earths and LREE relative to actinides. Differences between trace-element patterns of Independence volcanics and orogenic andesites argue against significant involvement of a subducted slab in the genesis of the rocks considered here. Rather, trace-element characteristics are interpreted to reflect those of subcontinental mantle of the Beartooth region.

Mantle-derived rocks have (87Sr/86Sr)i of 0.7046–0.7054, (143Nd/144Nd)i of 0.5114–0.5119, and (206Pb/204Pb)i of 16.0–17.4, values that are consistent with the trace-element enrichments and with ancient ages of enrichment events. Pb isotopic compositions display correlations of 207Pb/204Pb and 206Pb/204Pb interpreted to be secondary isochrons recording ages of mantle enrichment, 3.8 and 2.0 b.y. Pre–3.8-b.y. mantle had µ = 9.75.

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